The present invention relates in general to diode strings and relevant ESD protection circuits. More particularly, it relates to diode strings and ESD protection circuits characterized by low current leakage during normal operation.
Among ESD protection devices, proper forward biasing of a diode during an ESD event allows only a small silicon area to be required for effective ESD protection.
A conventional diode string consists of several diodes connected in series. When coupled between high-voltage and low-voltage power lines, the conventional diode string acts as an ESD protection circuit, clamping the voltage across the power lines and protecting devices from high voltage stress. Nevertheless, a parasitic Darlington amplifier may be formed in the diode string by series-connected parasitic bipolar junction transistors (BJTs), resulting in constant substrate current leakage forward to a substrate. This substrate current leakage becomes more severe as operating temperature or diode count in the diode string increases.
Conventional solutions to the leakage problem include adding extra circuitry to reduce the current gain of the Darlington amplifier, or physically elimination of the Darlington amplifier.